Secondary batteries are batteries that undergo repeated charging and discharging by chemical energy and electric energy being interconverted through the chemical reactions of oxidation and reduction. Secondary batteries generally include four basic elements: an anode, a cathode, a separator, and an electrolyte. Herein, the anode and the cathode are collectively referred to as an electrode, and among the elements forming electrode materials, a material causing the actual reaction is referred to as an active material.
Generally, lithium ion secondary batteries use a liquid electrolyte and an electrolyte including a liquid. However, liquid electrolytes have disadvantages in that they are volatile, and therefore, may present an explosion hazard. In addition, the liquid electrolytes have inferior thermal stability.
Meanwhile, solid state batteries using a solid state electrolyte have low danger of explosion, and also have excellent thermal stability. In addition, when a bi-polar plate is used, high operating voltage may be obtained by a series connection through electrode lamination, and in this case, higher energy density may be obtained compared to the energy density in a series connection mode of cells using a liquid electrolyte.
In order to prepare such an all solid state battery, a solid electrolyte transferring lithium ions is necessary. Solid electrolytes are largely divided into an organic (polymer) electrolyte and an inorganic electrolyte. The inorganic electrolyte is further divided into an oxide-type electrolyte and a sulfide-type electrolyte.
Of these, the oxide-type solid electrolyte is an oxygen-including electrolyte, such as a LiPON-type, a perovskite-type, a garnet-type and a glass ceramic-type. Oxide-type solid electrolytes have an ionic conductivity of 10−5 to 10−3 S/cm lower than sulfide-based electrolytes. The oxide-type solid electrolyte, however, has advantages in that the oxide-type solid electrolyte is stable with respect to moisture and the reactivity in the atmosphere due to oxygen is low, as compared to the sulfide-type solid electrolyte.
The oxide-based solid electrolyte has high grain boundary resistance, and therefore, an electrolyte membrane or pellets, in which necking between the particles are formed from high temperature sintering, may be used, and there is a problem in that mass productivity is very low to form a large-area electrolyte membrane since high temperature sintering is carried out at a temperature of 900 to 1400° C.
Particularly, a garnet-type solid electrolyte requires a long time of 6 hours or longer at a high temperature of 1000 to 1200° C. in final calcination, and in order to prevent lithium volatilization, and to secure phase changes and composition uniformity, pellet-covered garnet may be used. However, there is a disadvantage in that the proportion of garnet secured using such pellets is usually less than 20% by weight with respect to the total weight, which is a very small amount.
The US Patent Application Publication No. 2013-0344416 discloses solid oxide ceramics prepared by hot pressing pellets that are prepared including lithium carbonate, lanthanum hydroxide, zirconium oxide, and alumina; however, there is a disadvantage in that pellet-type LLZ with low crystallizability is formed.
Accordingly, in order to secure a large amount of garnet powder, much research has been conducted, including basic physical property studies, preparation of garnet sheet, preparation of a complex solid electrolyte of garnet and polymers, and studies on the materials capable of being utilized in the manufacturing process of all solid state batteries.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.